Soda lime silica glass has many desirable properties in a wide range of conditions including, for example, good transparency and clarity, high durability, etc. However, in some cases, the degree of mechanical strength of soda lime silica glass may depend upon the presence of (i) flaws in the glass originating at fabrication; and/or (ii) surface flaws that develop as the surface of the glass corrodes over time, which may cause a so-called “blistering” at the glass surface. These surface flaws sometimes may become more significant as the surface area to volume ratio of a piece of glass increases (e.g., for thinner articles of glass). While sodium dioxide (NaO2) may be used to help reduce the melting point of glass during its manufacture in certain circumstances, the presence of Na+ ions in a glass article (and particularly toward the surface/near surface area of the glass article) may have a negative impact on the chemical and/or mechanical durability of the article in some cases.
For example, in certain cases, Na+ ions may cause soda lime silica glass to deteriorate in quality. This may occur in a two-stage process in some cases. The first stage may be an ion-exchange process between H+ and/or H3O+ ions from moisture that penetrates the surface of the soda lime silica glass and an alkali metal ion (e.g., Na+) that is removed from the glass. At this stage, the silica network may remain unchanged, but an alkaline film of or including NaOH and H2O may form at the glass surface. In some examples, the film may become increasingly alkaline, as more moisture penetrates the surface of the glass and reacts with the sodium ions near the surface of the glass. In certain cases, if and/or when the pH reaches approximately 9 or higher (e.g., as the film becomes increasingly basic), the second stage may occur. The second stage may include decomposition of the silica network, which may lead to the formation of nano-cracks that may, in turn, potentially weaken the glass substrate.
Furthermore, glass strength may sometimes be controlled by the size of the “worst” or most severe defect (e.g., cracks and/or nano-cracks), which may vary from sample to sample. In certain cases, the surface nano-cracks and/or other flaws in soda lime silica glass (e.g., created by the reaction between Na+ ions and moisture and/or those present because of other factors) may act as “stress concentrators.” In some cases, less stress may be needed for the crack to become bigger, spread, and/or break the glass substrate because these nano-cracks are present in the first place. Additionally, because these cracks may form near the surface of the glass (e.g., because of the reaction of sodium ions and moisture to form an alkaline “coating” near the surface of the glass), the surface of the glass may be particularly prone to damage from lower amounts of stress than normal (e.g., more prone than a crack-free portion of soda lime silica glass).
In some general cases, the larger the crack, the lower the stress required for the crack to propagate. The potentially large number of flaws may cause the glass to fracture more easily than it normally would. In some cases, unstrengthened glass may fracture at stresses that are 5, 10, or even 100 times lower than the theoretical strength of the glass, e.g., because of the presence of cracks/nano-cracks. This may make it difficult to accurately predict the amount of stress a glass substrate may be subjected to before the glass will crack, fracture, and/or break. This variability in the strength of glass substrates having similar compositions and thicknesses may lead to increased accidental breakage and/or increased production costs, e.g., related to having to produce a glass substrate of higher strength than necessary to be “safe.” Indeed, there may be about 25% variability in the stresses that could cause glass substrates of similar compositions and thicknesses to crack, fracture, and/or break.
In other words, the amount of stress required to cause different glass substrates of similar composition and thickness to crack, fracture, and/or break may vary. The amount of stress that causes one substrate to fracture may not fracture a second substrate of similar composition and thickness. In some cases, it may be necessary to increase the thickness of glass because of the possible combination of low overall strength and high strength variability, if glass is to be used at all. Thus, in some cases, the use of soda lime silica glass as an engineering material may be limited by issues arising from these flaws, such as its brittle fracture behavior, strength variations, and/or its low effective strength under normal use conditions.
As the application of thin glass becomes more widespread (e.g., in the electronics market), factors such as mechanical hardness, resistance to marring and scratching, as well as thermal stability, become potentially more important considerations.
Thus, it will be appreciated that there is a need for improving the surface and/or near-surface properties of glass so that it may be sufficiently durable while maintaining and/or improving its other desirable properties (e.g., electronic grade, transparency, etc.).
There currently are several methods that may be employed to strengthen soda lime silica glass, namely, thermal (e.g., mechanical) heat treatment (e.g., heat strengthening and/or thermal tempering) and chemical tempering. Heat treating, for example, typically involves heating a glass substrate, and cooling the hot surface more rapidly than the interior. This creates compressive stress near the surface, which is balanced and/or offset by tensile stress toward the interior. Chemical tempering, on the other hand, involves an ion-exchange process. In certain chemical tempering implementations, larger ions are substituted for smaller ions at the surface of the glass. This process is sometimes referred to as “ion stuffing.” Both techniques may induce a compressive stress in the surface of the glass substrate in some instances.
A stress profile shows the relative amount and type of stress present in the substrate at various points. FIGS. 1(a)-(b) are illustrative stress profiles for each of the two techniques described above. More particularly, FIG. 1(a) illustrates a cross-section of a glass substrate, showing the residual stress profile caused by thermally strengthened glass.
As discussed above, heat treating (e.g., heat strengthening and/or thermal tempering) for glass involves strengthening the glass by altering the stress of the glass. In some cases, thermal tempering methods will build up and/or increase a residual compressive stress state at the surface of a glass substrate, and up to a certain depth below the surface. This residual compressive stress state is equilibrated and/or balanced by a tensile stress state in at least some of the internal portions of the glass. FIG. 1(a) illustrates the compressive stress at the surface and/or near-surface portion(s) 100(a) of substrate 1(a), and the tensile stress toward the interior portion 101(a) of substrate 1(a). As can be seen from FIG. 1(a), the stress of interior portions of a glass substrate that has been mechanically strengthened (e.g., heat treated) may be tensile, in certain cases.
FIG. 1(b), on the other hand, illustrates the cross-section of a glass substrate that has been chemically tempered. In FIG. 1(b), while substrate 1(b) has compressive stress in areas 100(b) at/near its surface, the compressive stress does not penetrate as deeply into the surface, and the tensile stress in the interior portion 101(b) of the glass substrate 1(b) is lower than glass substrate 1(a) that was thermally tempered. When a glass substrate is chemically tempered, the composition of the surface and/or near-surface regions of the substrate is changed. Na+ ions may be removed in certain cases, and thus the silica network of the glass substrate generally will not be as susceptible to damage related to reactions between Na+ ions and external moisture.
A disadvantage of thermal tempering is that in some cases it may not be used effectively for thin glass (e.g., glass that is approximately or less than 1.5 mm in thickness) because it may cause surface wrinkling and/or warping. Further, thermal tempering may sometimes lose efficacy as the glass gets thinner. In other examples, thermal tempering may not achieve the same level of “temper” as in chemical tempering. However, chemical tempering may take a very long to sufficiently strengthen a glass substrate (e.g., hours as opposed to minutes).
Thus, although thermal and chemical tempering may be advantageous in certain instances, those skilled in the art will appreciate that there is a need for faster ways to better strengthen a glass substrate (e.g., for thick and thin substrates).
In certain example embodiments, a method for increasing the strength of a glass substrate is provided. A plasma is struck using at least one plasma source and first and second electrodes disposed on opposing major surfaces of a glass substrate, wherein the plasma comprises replacement ions. The replacement ions are driven into the opposing major surfaces of the glass substrate so as to increase the strength of the glass substrate.
In certain example embodiments, a method of using plasma to strengthen a glass substrate comprising sodium ions is provided. A plasma is struck using at least one plasma source and first and second electrodes disposed on opposing major surfaces of a glass substrate, with the plasma comprises positive ions. An electric field is used to drive the positive ions into the at least one major surface of the glass substrate so as to replace at least some of the sodium ions and increase the strength of the glass substrate.
In certain example embodiments, a chemically-strengthened glass article comprising soda lime silica glass is provided. The article comprises at least one of potassium, lithium and magnesium plasma-implanted replacement ions in a surface region of the glass article. The surface region extends from a major surface of the glass article to a depth of at least about 5 microns, more preferably at least about 7 microns, and sometimes at least about 50 microns. At least some of the replacement ions have replaced sodium ions such that the glass article has fewer sodium ions than a glass article that has not been chemically strengthened. The glass article has a strength of at least about 200 MPa, and more preferably at least about 400 MPa.
In certain example embodiments, a method for increasing the strength of a glass substrate is provided. At least one plasma torch comprising at least first and second electrodes is disposed on at least a first major surface of a glass substrate. A plasma comprising replacement ions is sprayed through a nozzle of the plasma torch via an applied electric field between the two electrodes such that the plasma is sprayed proximate the first major surface of the glass substrate. The replacement ions are driven into the at least one major surface of the glass substrate so as to increase the strength of the glass substrate.
In certain example embodiments, a method for strengthening a soda lime silica glass substrate is provided. First and second plasma torches or arc jets are disposed on opposing major surfaces of a glass substrate. A plasma comprising replacement ions is sprayed onto the opposing major surfaces of the glass substrate via each plasma torch or arc jet. The replacement ions are driven into the first and second major surfaces of the glass substrate by virtue of electric fields between the first and second electrodes of each plasma torch or arc jet, so as to increase the strength of the glass substrate.
In certain example embodiments, a method of making a glass substrate is provided. Opposing major surfaces of a soda lime silica glass substrate are exposed to plasmas containing ions, the soda lime silica glass substrate at least initially including 10-20 wt. % Na2O. Electrodes associated with the plasma are selectively activated to drive the ions, directly or indirectly, into surface regions of the glass substrate and force sodium ions out from the surface regions to reduce Na2O content of the glass substrate to less than 10 wt. %.
In certain example embodiments, a method of making a glass substrate is provided. A plasma is struck in a tin bath section of a float line at least over a molten glass ribbon, with the plasma acting as a positive electrode and the tin bath acting as a negative electrode. Sodium ions are driven out of the molten glass ribbon and into the tin bath via an electric field created by the positive and negative electrodes and at least partially present in the molten glass ribbon. The glass substrate is allowed to be formed, with the glass substrate having less than 20 wt. % Na2O.
In certain example embodiments, a method of making a glass substrate is provided. A molten glass ribbon is provided in a tin bath portion of a float line via at least one plasma. Sodium ions are driven out of the molten glass ribbon and into the tin bath so as to reduce the sodium ion-content of the molten glass ribbon. The glass ribbon is maintained at one or more temperatures high enough such that the glass ribbon remains in a molten state, even as the composition of the glass ribbon changes, in making the glass substrate.
In certain example embodiments, a strengthened glass substrate comprises a silicate matrix, wherein the matrix comprises at least some argon atoms and wherein the glass substrate is substantially depleted of sodium ions and has a strength of at least about 600 MPa, and more preferably at least about 1000 MPa.
In certain example embodiments, a method of making a glass substrate is provided. Alumina is driven into molten glass, and sodium is forced out of the molten glass, via at least one plasma including alumina. The molten glass is pulled in making the glass substrate.
In certain example embodiments, a strengthened glass substrate comprises less than 10 wt. % Na2O and at least one type of ion selected from the group consisting of potassium, lithium and magnesium ions. At least some of the ions have replaced sodium ions originally present in a glass ribbon leading up to the glass substrate. The glass article has a strength of at least about 400 MPa.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.